The present invention relates to electrophotographic photoreceptors. More particularly, the present invention relates to an organic electrophotographic photoreceptor having improved durability against active gases (such as ozone), as well as enhanced photoreception and repetition characteristics.
Among conventional devices, electrophotographic photoreceptors (referred to as "photoreceptors" below) are generally used in such applications as copiers and printers. These known uses for photoreceptors feature devices which consist mainly of an inorganic photoconductive material such as selenium, selenium alloy, zinc sulfide, or cadmium sulfide.
Many photoreceptors of organic photoconductive material have recently been developed having advantageous characteristics. These organic photoconductive materials capitalize upon improved properties including flexibility, heat stability, film formation capability, light weight, and low costs.
Prominent among organic photoconductive materials are function-separated laminated organic photoreceptors. Generally speaking these materials include a photosensitive layer which is divided into a charge generation layer (for receiving light to generate charge carriers) and a charge transport layer (for transporting charge carriers generated).
Function-separated laminated organic photoreceptors additionally provide improved characteristics when the respective layers are formed of an material suitable for corresponding functions and combined. Such photoreceptors may be combined, for example in a negatively-charged function-separated laminated organic photoreceptor. This type of function-separated laminated organic photoreceptor generally includes a charge generation layer and a charge transport layer laminated on a conductive substrate. The layers are arranged in this particular order for generating a photosensitivity comparable to that of selenium photoreceptors.
Electrophotographic devices are used for image formation processes, which are repeatedly performed upon their surfaces. Cycles of these image formation processes include charging, exposure, development, transfer, cleaning, and erasing. Since these image formation processes must be repeatedly done on the surface of photoreceptors, the photoreceptors must exhibit high levels of stability. This stability is required to last over time throughout repeated cycles of the image formation processes, numbering in the thousands.
Prior art photoreceptors have sought improved productivity, for operation over repeated cycles. However, prior an photoreceptors have failed to provide high quality images during repetitive processes. Consequently, the overall quality of the reproduced image steadily declined over the time with known devices.
It is further noted that repetitive processes such as those outlined above cause degradation of the charge potential which substantially increases the residual potential, resulting in inferior images.
The decrease in the overall quality is attributed to numerous factors. A primary factor is corona discharge. When photoreceptors are used in photocopying machines, they repeatedly contact noxious gases, such as ozone, generated by corona discharge.
Ozone gas in particular has been implicated in adversely effecting the function of photoreceptors in photocopying machines. NO.sub.x is another gas which has been implicated in detrimentally impacting overall quality and life span of conventional photoreceptors used in photocopying machines.
In recent years there has been a growing demand for photoreceptors which provide faithful reproductions without compromising the quality of the image. Demand has also fueled an interest in photoreceptors which are capable of providing high quality images at minimal cost and that retain their imaging properties over time and repeated use.
It is also required that the particular photoreceptors exhibit improved electric characteristics, showing stability in respective decreases in charge potentials. In general, charge generation layers absorb light and generate pairs of charge carriers. Such generated charge carriers must then be transported quickly and injected into conductive substrates or into a charge transport layer. This process must be accomplished quickly before the charge carriers are lost to recombination or charge traps.
It thus is preferable that the charge generation layer have adequate thinness and an even and consistent width. Conventional photoreceptors have a charge generation layers measuring less than one .mu.m. However, prior art photoreceptors with thin film charge generating layers are plagued with numerous disadvantages.
Prior art disclosures include the addition of various photodeterioration agents to prevent deleterious changes in surface potential. Use of various antioxidants (for example, tri-akylphenol derivatives or di-lauryl thiopropionates) is known in prior art photosensitive layers. Other organic phosphites have been added for reducing or preventing degradation of prior art photosensitive layers.
Attention is called to the following publications which suggest the use of specific compounds in order combat degradation of conventional photosensitive layers. These publications include, Japanese Laid Open Patent Publication No. 62-234164, Japanese Laid Open Patent Publication No. 62-105151, Japanese Laid Open Patent Publication No. 1-118137, and Japanese Laid Open Patent Publication No. 5-257299.
Japanese Laid Open Patent Publication No. 62-234164 teaches the use of an organic phosphite compound which is incorporated into either the charge generating or charge transfer layer. However, this disclosure is innocently silent of any disclosure suggesting the use of a hindered phenol in combination with an antioxidant as used in the present invention.
Similarly, Japanese Laid Open Patent Publication No. 62-105151, Japanese Laid Open Patent Publication No. 1-118137, and Japanese Laid Open Patent Publication No. 5-257299 envision using various conventional inorganic compounds incorporated within photosensitive layers for preventing changes in surface stability of the photoreceptors.
Japanese Laid Open Patent Publication No. 1-118137 suggests using a polycarbonate in conjunction with conventional hindered phenols in order to maintain consistent electrical characteristics. This disclosure, however, is devoid of teachings incorporating an antioxidant as used in the present invention.
Likewise, Japanese Laid Open Patent Publication No. 5-257299 proposes using additional binder resins made of specified copolymers in order to adjust the electrical resistance and to resist mechanical changes in, the photosensitive layer. However, as among other conventional disclosures, the conventional binder component is combined with known hindered phenols or amines. Consequently, the benefits of combining an ester phosphite antioxidant and a hindered phenol antioxidant, as taught by the present invention, are not procured.
Unfortunately, as discussed above, of prime concern with organic photoreceptors are the charge potentials which are reduced during repetitive use. This causes degradation of charging characteristics, resulting in degraded image quality for images produced with such photoreceptors. Degradation of such charging characteristics is mainly due to the degradation of organic material on the surface of photoreceptors caused by active gases, such as ozone (O.sub.3). These active gases are generated by corona discharge from a charger, and continue to provide a problem among known disclosures.
In other prior art attempts to solve this problem, fans have been installed for ventilating the ozone. The fans remove ozone to reduce the amount of ozone contacting surfaces of photoreceptors.
Alternatively, various antioxidants such as 2,6-di-butyl-4-methylphenol (BHT), pentaerythritol-tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], 2,4-bis-(n-octylthio)-6-(4-hydroxy-3,5-di-t-butylanilino)-1,3,5-triazine have been added to outermost surfaces of photoreceptors to prevent such degradation.
However, neither employing such conventional ventilation means, nor use of antioxidants has sufficiently protected photoreceptor surfaces from ozone driven effects. To date, nothing known among the prior art has adequately addressed this longstanding problem of degradation of charging characteristics. Fan-equipped devices have proven to be prohibitively expensive and addition of an antioxidants likewise is problematic.
Thus a clear need exists to prevent active gases from adversely affecting surfaces of photoreceptors and causing such photoreceptors to have degraded charging characteristics. In addition, known organic photoreceptors still have not managed to satisfy the market's requirements for repetition characteristics.